Typetester

Typetester is a brilliant online tool for web designers, allowing them to choose fonts based on what they will actually look like in a browser, rather than trying to guestimate the appearance in Photoshop by disabling antialiasing, etc.

Web typography is generally rather unimaginative due to the cross-platform constraints, but with typetester you can easily pick the best fonts for each platform and a default to fall back to, instead of just going with Verdana, Arial or Trebuchet, or doing everything with images as people have tended to in the past.

Cocotron

Cocotron is a port of the Cocoa libraries to Windows, effectively allowing cross-compilation of Cocoa applications to Windows.

It's not perfect, but if this takes off it could do for Cocoa what Mono has done for .NET - turning it from a proprietary single-platform development API to a sensible choice for cross-platform development.

Glen and Ken Aspeslagh from Ecamm have written a postmortem of their experiences using Cocotron to port their FileMagnet app to Windows.